Treatment of iron-cobalt alloys



atent ()fiiice 3,065,118 Patented Nov. 20, 1962 The present invention relates to iron-cobalt alloys, and more particularly to a process of treating magnetic ironcobalt alloy laminations to render them suitable for use in electromagnetic apparatus and to facilitate their processing for this purpose.

Iron-cobalt alloys have long been known in the art to be suitable for electromagnetic structures, such as cores in alternating current electrical apparatus. Difficulty has been encountered in the past, however, in the processing of such alloy laminations due to their brittleness, especially when the material is subjected to certain heat treatments for improving their magnetic properties. Prior attempts to overcome this problem have not been entirely successful, either because they were excessively time consuming or produced improved ductility only at the expense of optimum magnetic properties, or the proposed measures were not effective when applied to iron-cobalt alloys of high purity.

It is an object of the invention to provide an improved process of treating magnetic iron-cobalt alloys which avoids the above disadvantages.

It is another object of the invention to provide a simple and economical process for improving the ductility of high purity magnetic iron-cobalt alloy laminations to facilitate their processing for electromagnetic use while preserving the optimum magnetic and electrical properties thereof.

It is a specific object of the invention to provide a method of treating iron-cobalt alloys of the above type,

especially of high purity, wherein ductility of the alloy is ensured subsequent to the use of a hydrogen protective atmosphere which would otherwise cause embrittlement of the metal.

Other objects and advantages will become apparent from the following description and appended claims.

Briefly, the invention in its broad aspects comprises, in a method of treating an iron-cobalt alloy which is subjected to a heat treatment in a hydrogen atmosphere followed by a quenching treatment, the step of removing the hydrogen occluded in the iron-cobalt alloy in order to maintain or restore the ductility thereof necessary for cold rolling the alloy material.

The iron-cobalt alloy involved in the present invention is of the type disclosed in US. Patent 1,862,559, a typical composition being about equal proportions of cobalt and iron and about 2% vanadium, the percentages herein given being by weight. The alloy composition may, however, vary in the range of 30 to 70% cobalt, 1 to 3% vanadium, and the remainder iron. It will be understood that other materials such as chromium, carbon, nickel, manganese, and other elements may be present in minute quantities which, for all practical purposes, do not materially affect the properties of the described alloy composition.

The iron-cobalt alloys herein referred to are of the than 25 mils, a step which is necessary to develop good type produced by a vacuum-melting process and are, hence, of greater purity than the air-melted types which contain appreciably greater quantities of impurities such as carbon, hydrogen, nitrogen and other constituents. The

method of the present invention is of particular benefit in the processing of the high purity alloy containing practically no constituents other than cobalt, iron and vanadium, as compared to the impure, air-melted types of iron-cobalt alloy compositions.

In the process of treating the iron-cobalt laminations to prepare them for use in electromagnetic apparatus, the laminations must be cold-rolled to a final gauge of less magnetic properties in the strip. In order to make the material sufficiently ductile for the cold-rolling step, it has been necessary to quench the previously hot-rolled band from a temperature of upwards of 700 C. This procedure is particularly essential in the case of the high purity iron-cobalt alloys described above. Experience has shown that if the metal is allowed to cool slowly after such a heat treatment rather than immediately quenched, the alloy becomes embrittled probably due, theoretically, to a process of atomic rearrangement known as ordering. As has been indicated, such a quenching treatment is even more important in the case where high purity iron-cobalt alloys are being processed than in the case of relatively impure alloys of this type, e.g., those containing relatively high carbon contents. Apparently, such contained impurities themselves serve to prevent or retard the undesired ordering mentioned and less rapid cooling of the impure alloys has little or no adverse effect on the degree of ductility achieved therein.

A prior procedure of the above quenching type has involved reheating the hot rolled band in the presence of air and then quenching in water. A problem presented by such a procedure, however, is that the'high purity of the alloy, which is very important in obtaining good electrical and magnetic properties in the final product,

is not adequately protected by the air atmosphere, or, in fact, from contaminants contained in such atmosphere. We have further found in connection with the invention that in the use of a quenching treatment for high purity iron-cobalt alloys, the atmosphere in which the alloy is heated prior to quenching has a marked and unexpected elfcct on the ductility of the alloy. We have found, in particular, that while a hydrogen atmosphere provides effective protection against contamination of the iron-cobalt alloy, the alloy is on the other hand severely embrittled by water quenching after being heat treated in a hydrogen atmosphere. From the standpoint of maintaining the high purity of the alloy the use of hydrogen is especially desirable where large quantities of the metal are subjected to the heating process. In large scale commercial production, the metal band is conventionally heat treated in the form of convolutely wound coils, and in such form the material requires extended heating periods, e.g., several hours, in order for all parts of the coil to attain the desired temperature. The presence of a protective hydrogen atmosphere during such prolonged heating is essential because of the considerable risk, under such conditions, of oxidation and contamination of the alloy by atmospheric constituents such as oxygen, carbon, sulfur, and other elements.

In accordance with the invention, we have discovered that if appropriate steps are taken to remove the hydrogen which apparently permeates and becomes occluded in the alloy material during such exposure, considerably greater ductility is imparted to the iron-cobalt alloy than if the hydrogen is not removed. In one embodiment of our invention, a process is provided which comprises heating the iron-cobalt alloy to a temperature in the range of 7001000 C., and preferably in the range of about 800850 C., in a hydrogen atmosphere, for a period sufiiciently long for all the material to attain the stated temperature, quenching the thus heated alloy in a cold liquid quenching medium, reheating the alloy to a temperature of about 2003'00 C. in a non-hydrogen bearing, non-carburizing atmosphere such as air, vacuum, nitrogen, argon, or other preferably inert gas for a suificient period to allow diffusion of the occluded hydrogen out of the alloy, and then cooling the alloy either slowly or rapidly and with or without a quenching process, as desired.

In another embodiment of the present process, the ironcobalt alloy is heated initially as in the previously described embodiment to a temperature of 700 to 1000 C., and preferably in the range of 800-850" C., in a hydrogen atmosphere for a sufiicient period for all parts of the material to attain the stated temperature. Then the hydrogen is fully replaced during the heat treatment by a different, non-hydrogen bearing, 7 non-carburizing atmosphere of the types mentioned above, and the heating of the alloy material is continued in the replacement atmosphere a for short period, e.g., -l5 minute's. Immediately thereafter the alloy material is quenched in a suitable cold quenching medium.

In still another embodiment of the invention, the ironcobalt alloy is heated initially in a hydrogen atmosphere to the elevated temperature ranges and for the period described in the previous embodiments, and then quenched in a suitable cold liquid quenching medium. Thereafter, the alloy material is reheated until it reaches a temperature of between 700'l000 C., and preferably of around 800850 C., in a nonahydrogen bearing, noncarburizing atmosphere of the types set forth above, and then quenched in a cold liquid medium.

Of the three embodiments described, the second embodiment, i.e., the process wherein the hydrogen in the initial heat treatment is replaced by a non-hydrogen bearing atmosphere prior to quenching, is preferred for the reason that the quenching of hydrogen-containing alloy material is in the other two embodiments involves the slight possibility that cracks may be produced in the metal strip due to the sudden quenching of the hydrogenembrittled material.

In those cases where an oxygencontaining gas such as air, or other possibly reactive gas, is used as the nonhydrogen bearing gas in the above processes, exposure of the alloy thereto should be limited to no greater period than is necessary, so as to avoid undesired contamination of the alloy material.

The quenching medium used for the above processes is preferably an iced brine solution, but it will be understood that other known quenching liquids or gases of suitably low temperature may be used if desired. A temperature of about 0 C. for the quenching medium has been found satisfactory, but this may vary widely.

Any of the above treatments as described above when used after a conventional hot rolling process results in a suitably ductile, hot rolled band of iron-cobalt alloy which may be subsequently subjected to a cold rolling process for reducing the band thickness to the desired final gauge. Thereafter, the band or laminations may be treated in accordance with known practice to further develop the magnetic and other properties of the alloy.

In a series of experiments made in connection with the invention, a number of sample strips 4% inches long, /8 inch wide and .072 inch thick were prepared from a hot-roller band of an alloy composed of 49% iron, 49% cobalt and 2% vanadium. These samples were subjected to a heat treatment as follows:

The samples were then tested for ductility by gripping the ends with hand vises and bending. The radius to which the samples could be bent without cracking or fracturing was used as a measure of ductility. Therefore, the smaller the radius to which the sample could be bent, the more ductile the material.

Of the above samples, four were quenched from a hydrogen atmosphere after 1 /2 hours at, respectively, 750 C., 770 C., 800 C., and 860 C. The sample heated at 750 C. cracked when bent to a minimum radius of inch, the sample heated at 770 C. cracked when bent to a radius of inch, the one heated at 800 C. fractured at a radius of /2 inch, and that heated at 860 C. fractured at a radius of 2% inches.

Four other samples respectively treated at the same temperatures but in the presence of an air atmosphere showed no cracks at all after being bent to a radius of inch.

These comparative tests demonstrated that brittleness is imparted to the alloy by treatment with hydrogen, and the higher the temperature, the more severe was the embrittlement. This is in contrast to the retention of ductility by the air-exposed samples regardless of the temperatures used.

Other samples of the same alloy composition were quenched from a hydrogen atmosphere after 1 /2 hours at 860 C. and then were reheated in different, nonhydrogen atmospheres to determine the effects produced thereby. In one case the reheating was to a temperature of 860 C. in an air atmosphere for 15 minutes, followed by a quench. In the other case the reheating was to a temperature of 200 'C. for 2 hours in a vacuum. Upon bending thg thus treated samples to a radius of 7 inch, no cracks were found therein.

It is evident from the above experiments that the subsequent reheating step in which no hydrogen was used was effective to restore ductility to samples which had previously been exposed to a hydrogen heat treatment.

The invention accordingly makes it possible to combine the advantages of using a hydrogen protective atmosphere in treating iron-cobalt alloys with the need for employing a rapid quench of a heat treated alloy band, and there is thus obtained the optimum ductility of the alloy band to facilitate subsequent working of the material without sacrifice of its good electrical and magnetic properties.

While the alloy is used for the purposes of the invention chiefly in the form of strips, sheets, laminations or other laminar form, the alloy metal may be processed if desired in the form of bars, rods and other shapes in accordance with the principles of the invention to obtain corresponding benefits.

It will be understood that the expression iron-cobalt alloy is used in the claims in a general sense, and that it is intended to include thereby the alloy materials described in detail hereinabove.

While the present invention has been described with reference to particular embodiments thereof, it will be understood that numerous modifications may be made by those skilled in the art without actually departing from the scope of the invention. Therefore, the appended claims are intended to cover all such equivalent variations as come within the true spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. The method of treating high purity magnetic ironcobalt alloy material consisting essentially of 30-70% cobalt, 1-3% vanadium and the remainder iron to adapt it for cold rolling which comprises heating said alloy material in a hydrogen atmosphere at a temperature of 700-l000 C., quenching the thus heated alloy material to avoid embrittlement by ordering, reheating the alloy material to a temperature of about 700-1000 C. in a non-carburizing atmosphere free of hydrogen, and thereafter quenching the heated alloy material to avoid embrittlement by ordering.

2. The method of treating high purity magnetic ironcobalt alloy material consisting essentially of 3070% cobalt, 13% vanadium and the remainder iron to adapt it for cold rolling which comprises heating said alloy material in a hydrogen atmosphere at a temperature of 7001000 C., replacing the hydrogen atmosphere during said heating with a non-carburizing atmosphere free of hydrogen, and quenching the thus heated alloy material to avoid embrittlement by ordering.

3. The method of treating high purity magnetic ironcobalt alloy material consisting essentially of 30-70% 20 cobalt, 1-3% vanadium and the remainder iron to adapt it for cold rolling which comprises heating said alloy References Cited in the file of this patent UNITED STATES PATENTS 1,862,559 White et al June 14, 1932 2,292,191 Brandt et a1. Aug. 4, 1942 2,591,460 Morrill Apr. 1, 1952 2,717,223 Binstock et a1. Sept. 6, 1955 2,905,547 Yoblin Sept. 22, 1959 OTHER REFERENCES Stanley: Metallurgy and Magnesium, American Society for Metals, 1949, pages 99-103, page 100 particularly relied on.

Gases in Metals, Sims, page 172-188, by American Society for Metals, 1953. Call No. TA460A44g.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent Non 3 O65 ll8 November 20 1962 Hans Wo Wawrousek et ale It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 3 line 31 for "a for" read for a line 4L9 for "is" read as line 57 after "necessary" and before the comma insert to remove the occluded hydrogen in the alloy column 4 line l forhot-roller read hot rolled Signed and sealed this 18th day of June 1963o (SEAL) Attest:

ERNEST w. SWIDER DAVID ADD Attesting Officer Commissioner of Patents 

1. THE METHOD OF TREATING HIGH PURITY MAGNETIC IRONCOBALT ALLOY MATERIAL CONSISTING ESSENTIALLY OF 30-70% COBALT, 1-3% VANADIUM AND THE REMAINDER IRON TO ADAPT IT FOR COLD ROLLING WHICH COMPRISES HEATING SAID ALLOY MATERIALS IN A HYDROGEN ATMOSPHERE AT A TEMPERATURE OF 700-1000*C., QUENCHING THE THUS HEATED ALLOY MATERIAL TO AVOID EMBRITTLEMENT BY ORDERING, REHEATING THE ALLOY MATERIAL TO A TEMPERATURE OF ABOUT 700-1000*C. IN A NON-CARBURIZING ATMOSPHERE FREE OF HYDROGEN, AND THEREAFTER QUENCHING THE HEATED ALLOY MATERIAL TO AVOID EMBRITTLEMENT BY ORDERING. 